1. Field of the Invention
The present invention concerns filtering and more particularly filtering large mass flowrates of gaseous fluids passing at low speed through a layer of filter medium.
2. Description of the Prior Art
It is well known that the effectiveness of filtration depends principally on the nature and on the particle size of the filter medium employed and on the thickness of the horizontal or otherwise bed formed by the layer of filter medium.
The problem of maintaining this thickness with time becomes crucial in certain applications where it is a question of filtering a gas laden with particles before it is vented to the atmosphere. Any reduction in this thickness may result in a loss of filtration effectiveness and an unfavorable change in the characteristics of the filter employed, which represents a non-negligible hazard to the immediate environment of the filter system.
This is especially so in the case of safety installations equipping pressurized water type nuclear reactors. Although the probability of a serious accident is extremely low, an attempt is made to avoid any irreversible deleterious effect on the confinement function that could result from any excessive internal pressure rise: it thus appears necessary to provide installations connected to the reactor confinement vessel for the purpose of expanding and then filtering the gases given off in order to eliminate any risk of destruction of the confinement vessel.
It must be borne in mind that operating conditions in applications such as these are particularly severe. The gas mixture to be filtered in the event of an accident (comprising air, carbon dioxide, water vapor, carbon monoxide and aerosols) leaves the vessel at a temperature of 140.degree. C., at an absolute pressure of 5 bars and with a mass flowrate of as much as 3.5 kg/s. The gas mixture reaching the filter system with this high flowrate is first expanded (to an absolute pressure of 1.1 bars) and then filtered by a filter medium, the mixture having a high speed on the input and output sides of the layer of filter medium but passing through the filter bed at a low speed.
There have been numerous laboratory experiments to evaluate such situations on a reduced scale.
One particularly interesting laboratory experiment is reported in a publication from an international symposium held at the end of October 1985 at Columbus, Ohio in the United States.
As part of the PITEAS Research and Development program, a sand filter was developed in the laboratory and used in numerous experiments in which the characteristics of the gas to be filtered were varied.
By taking samples on the input and output sides of the filter system, these experiments made it possible to demonstrate the effectiveness in the laboratory of a sand filter medium having an average particle size (in the order of 0.7 mm) and a depth (approximately 800 mm) appropriate to the required characteristics.
The filter system used comprised a cylindrical glass enclosure with a diameter of approximately 1 m with an inlet tube at the top and an outlet tube at the bottom, these vertical tubes being coaxial with the main body of the enclosure. The enclosure contained a layer of fine sand constituting a horizontal bed with a depth of 800 mm. It was, of course, necessary to provide means for supporting the filter medium and means for collecting the filtered gas and conveying it to the outlet.
By way of supporting means there were placed under the layer of sand, in succession:
a 50 mm deep "support" layer of sand with an average particle size significantly greater than that of the filter medium (approximately 2 mm), PA1 a metal mesh having a mesh size smaller than the average particle size of the sand in the support layer, PA1 a galvanized steel grating supported mechanically by means of commercially available rolled sections.
By way of recovery means there was provided a free space in the bottom part of the enclosure, between the grating and the bottom wall from which the filtered gas outlet ran.
Attempting to extrapolate from this design of sand filter to a full scale filter system would encounter numerous disadvantages.
First of all, the presence of the supporting sand layer would complicate the operations involved in charging the filter, because guaranteeing a constant thickness of the filter medium would naturally imply guaranteeing a constant thickness of the support layer. Also, it would be impossible to check this thickness after charging the filter medium (the thickness of the support layer could only be verified locally). This is a first major disadvantage, since it is not possible to do without this support layer of precise thickness.
Then, the necessity for the free space at the bottom to recover the filtered gas would entail a non-negligible risk of the filter medium support system collapsing. This hypothesis cannot be eliminated in that it is impossible to be certain that the metal parts of the support structure will not eventually become corroded, due to phenomena of condensation or chemical interaction with the filter medium. Although such corrosion is highly improbable, it has to be allowed for; it entails the risk that the support structure for the filter medium will collapse in the medium- or long-term, resulting in the filter being ruined.
Consideration might be given to filling this free space at the bottom with sand to eliminate the risk of collapse, but this would have undesirable consequences in relation to the recovery of the gas (head loss, insufficient increase in speed at the outlet), the weight of the construction (an additional 70 tons, approximately) and, of course, cost (the cost of the sand itself, and of the necessarily strengthened mechanical support means).
Consideration might be given to adopting and adapting techniques used in water filters, as some such filters incorporate a granular material filter medium comparable with the sand used here.
However, these always include a solid supporting floor with a free space at the bottom, whether they are contraflow filters (with successive layers on a floor comprising a coarse gravel support layer, a coarse filter gravel layer and a double fine filter layer of sand) or simple water reflow washing filters such as Wheeler filters (which have a concrete floor in which are pyramid-shaped cells filled in with porcelain balls over which are laid successive layers with progressively reducing particle size to finish with a layer of fine sand). Thus these techniques do not provide any teaching directed towards elimination of the risk of the filter medium support structure collapsing. Moreover, adopting such filters for the target application would entail using a very high operating pressure in order to obtain a high speed at the outlet from the filter medium, and thus an enormous structure to accommodate such conditions.
An object of the present invention is to propose a filter system designed to overcome the aforementioned disadvantages through the use of a simple, relatively economical and reliable structure, especially with regard to the risk of collapse of the filter medium support structure.
Another object of the invention is to propose a filter system which makes it possible to obtain a high ratio between the speed of the gas in the recovery device and its speed in the filter medium, by which is meant a ratio of as high as 100:1 or more, for example.